This invention relates to a muffler for use in road vehicles. There are various methods of suppressing the level of noise generated by the engine etc. of vehicles, however, it is most effective to close the noise generating sources with covers. The engine and the fan forming main noise generating sources are enclosed by the engine compartment wall, however, the engine compartment is, in general, not covered up tight because it is required to radiate the heat generated by the engine per se. Lately, such noise has posed a social problem as a public hazard and it has become impossible to achieve the target of suppressing the noise by means of a conventional cover. Therefore, noise-proof or noise suppressing vehicles have such a structure as the engine compartment is covered up tight and the air within the engine compartment can be discharged through its muffler by means of an ejector.